A number of pathogenic bacteria, notably Mycobacteria and Corynebacteria, as well as saphrophytes such as Nocardia produce cell wall polysaccharides containing D-arabinose. Virtually nothing is known about the biosynthesis of D-arabinose although its evantiomorph L-arabinose is known to arise by decarboxylation of glucuronic acid (via UDP-glucuronic and UDP-xylose). Preliminary studies using 1-14C-glucose and 6-14C-glucose as tracers suggested that both isotopes could serve equally well as precursors to 14C-arabinose in the cell wall suggesting that hexose must first be degraded to trioses which were than somehow reassembled to form pentoses. In order to verify or negate this proposed pathway, we are growing cells for various times in the presence of various labeled substrates (14C-acetate, glycerol, alanine, ribose, mannose, glucose) in order to determine which is the best precursor of arabinose. We also plan to degrade the arabinose formed from various specifically labeled substrates (i.e., 1-14C-glucose, 6-14C-glucose, etc.) to determine the labeling patterns and thus the possible pathway of synthesis. BIBLIOGRAPHIC REFERENCES: Izumori, K., Rees, A., and Elbein, A.D. Purification, crystallization and properties of D-ribose isomerase of Mycobacterium smegmatis. J. Biol. Chem. 250, 8085 (1975). Izumori, K., Mitchell, M. and Elbein, A.D. Evidence that isomerization of D-ribose and L-rhamnose are catalyzed by the same enzyme. J. Bacteriol. 126, 553 (1976).